In collaboration with Dr. P.Y. Paterson we have encountered a factor BP-SS in the circulation of neonatal Lewis-strain suckling rats that inhibits the reaction of myelin basic protein (MBP) with anti-MBP, and we have determined that BP-SS exhibits a true immunological cross-reaction with MBP in its reaction with anti-MBP. During this coming year we will investigate whether BP-SS is synthesized by cells from the nervous system compartment, whether it represents only haptenic portions of fragmented MBP, or whether it comprises a neonatal circulating factor of non-nervous-system origin that contains determinants in common with MBP. The immunochemistry and in vivo reactivity of a highly purified anti-MBP antiserum, already raised, will be studied in depth; in particular, how it reacts with various myelin fractions and subfractions in vitro and with myelin components in vivo. A method for stabilizing myelin suspensions in vitro with minimal loss of native characteristics of myelin membrane surfaces will be sought. Purification and development of specific antiserum reagents for the seven classes and subclasses of rat immunoglobulins will be continued to provide reagents for differential radioimmunoassays.